1. Field of the Invention
The present invention relates to a fluid-filled vibration damping device suitably employed as engine mounts, body mounts, etc., for use in automotive vehicles, for instance.
2. Discussion of Related Art
In installing, on an automotive vehicle, a power unit such as engine as a vibration generating source, an engine mount as a vibration damping device is generally interposed between two members, i.e., between a body frame of the vehicle and the power unit, for instance. There is known a fluid-filled vibration damping device shown in FIG. 13 as one example of such an engine mount.
The fluid-filled vibration damping device shown in FIG. 13 includes: a shaft-like first mounting member 101; a cylindrical second mounting member 102 located radially outwardly of the first mounting member 101 with a suitable radial distance therebetween, in coaxial relation with each other; an elastic body 103 interposed between the first and second mounting members 101, 102 so as to connect those members 101, 102 integrally to each other; a diaphragm 104 which is retained at its peripheral portion by an inner circumferential portion of the second mounting member 102 and which cooperates with the elastic body 103 to define therebetween a fluid chamber 145 which is filled with a fluid L; and a partition structure 105 which includes an annular metallic partition member 151 and a flexible movable plate 152 disposed in a central opening of the metallic partition member 151 and which is retained at its peripheral portion by the inner circumferential portion of the second mounting member 102 so as to divide the fluid chamber 145 into a primary fluid chamber 146 and an auxiliary fluid chamber 147 that communicate with each other through an orifice passage 157.
The fluid-filled vibration damping device described above is installed on the vehicle such that the first mounting member 101 is fixed to a mounting portion of the power unit and the second mounting member 102 is fixed to a mounting portion of the vehicle body, whereby the power unit is suspended from the vehicle body in a vibration damping or isolating fashion.
When vibrations in a high frequency range are generated by actuation of the engine, for instance, with the fluid-filled vibration damping device installed as described above, the vibrations can be effectively absorbed owing to elastic deformation of the elastic body as a result of relative displacement of the first mounting member 101 and the second mounting member 103. Vibrations in a low frequency range such as engine shakes, on the other hand, can be effectively absorbed owing to resonance of the fluid L which flows between the primary fluid chamber 146 and the auxiliary fluid chamber 147 through the orifice passage 157 as a result of a fluid pressure variation in the primary fluid chamber 146 upon application of the vibrations.
In the fluid-filled vibration damping device described above, when the fluid pressure variation in the primary fluid chamber 146 rapidly increases by application of large vibrations thereto, the orifice passage 157 is placed in a clogged state, so that the fluid L does not flow therethrough. When the orifice passage 157 is placed in the clogged state, the negative pressure generated in the primary fluid chamber 146 immediately after the clogging of the orifice passage 157 also increases, so that a multiplicity of air bubbles are generated in the fluid L of the primary fluid chamber 146, causing a cavitation phenomenon (hereinafter may be referred to simply as “cavitation”). The air bubbles thus generated in the fluid L of the primary fluid chamber 146 undesirably cause abnormal noise upon disappearance or extinction by the subsequent pressure variation or impact, and the abnormal noise is undesirably transmitted to the vehicle cabin, causing a risk of deterioration in silence in the vehicle cabin and driving comfort as felt by a vehicle driver or passengers. The abnormal noise which results from the cavitation tend to be generated when large vibrations are input upon starting of the engine, during running on bumpy roads, etc.
In view of the above, there are proposed various fluid-filled vibration damping devices which are arranged to deal with the cavitation.
For instance, JP-A-2004-003634 proposes a fluid-filled vibration damping device adapted for absorbing or mitigating an impact wave generated upon disappearance of the bubbles by a coating rubber layer which is provided on a surface of an orifice-defining member at a primary-fluid-chamber-defining area. In the disclosed device, however, when the coating rubber layer is formed integrally with the orifice-defining member by injecting a rubber material for the coating rubber layer into a mold and vulcanizing the rubber material, the pressure required for the injection of the rubber material is high, thereby causing a risk of deformation of the orifice-defining member. Thus, the proposed device has a problem in its production.
JP-A-2004-190757 discloses a fluid-filled vibration damping device adapted for preventing growth of the air bubbles by providing a cushion surface which is opposed to an opening of an orifice passage to a primary fluid chamber with a suitable spacing distance therebetween. This arrangement, however, requires an additional member for providing the cushion surface, undesirably pushing up the cost of manufacture of the device.
JP-B-7-107416 and Japanese Patent No. 2805305 propose a technique to prevent occurrnce of the cavitation by providing a slit (a cut portion) at the middle of a partition member which partially defines a primary fluid chamber and an auxiliary chamber so as to provide a movable plate (elastic plate, or elastic partition wall) functioning as a valve, thereby preventing the fluid pressure in the primary fluid chamber from considerably lowered to negative pressure so as to avoid the occurrence of the cavitation.
In the disclosed technique, the movable plate having the slit (the cut portion) formed by cutting the movable plate in its thickness direction needs to assure sealing tightness at the slit to a certain degree of the fluid pressure. Since the adjustment between the fluid pressure and the sealing tightness is subtle, however, the disclosed technique may suffer from a difficulty in producing the movable plate.
In addition, in the movable plate functioning as the valve, the slit which has been placed in its open state by the fluid pressure needs to return to its closed state. The slit (the cut portion) may fail to close and remain in the open state due to subtle deformation of the slit or creep of the rubber that provides the movable plate as an elastic body, causing a problem of insufficient sealing tightness. Thus, even where the difference in the fluid pressure between the primary fluid chamber and the auxiliary fluid chamber is small, the slit (the cut portion) in the movable plate tends to be kept in the open state, making it difficult to assure good resonance effect based on the fluid flowing through an orifice passage.